This invention relates to a method of metallizing the surface of a phosphor screen for a cathode ray tube like a color television picture tube, more particularly, to the improvements in volatilizable substrates for the metal.
Phosphor screen for a cathode ray tube, for example, a tricolor mosaic screen for a color television picture tube, comprises phosphor dots or stripes arranged in a predetermind pattern on the surface of a glass face plate. The phosphor dots consist of dots of a blue-emitting phosphor, dots of a green-emitting phosphor and dots of a red-emitting phosphor. Formed on the surface of the phosphor screen is a so-called metal back, i.e., an electron-permeable metal layer like an aluminum layer. The metal layer acts as a light reflector and an electrical conductor. Since the surface of the phosphor screen is rough, the metal layer fails to have a specular smooth surface if formed directly on the surface of the phosphor screen, resulting in a decreased amount of luminescent light reflected by the metal layer toward the viewer. It follows that the light output of the cathode ray tube becomes insufficient, namely, the tube lacks in satisfactory brightness.
To solve the problem, attempts have been made to improve the method of metallizing the phosphor screen. A typical prior art in this line includes the steps of producing upon the surface of the phosphor screen a volatilizable substrate depositing a metal layer upon the substrate, and then volatilizing or backing out the substrate. U.S. Pat. No. 3,582,390 discloses a method of forming such a volatilizable substrate as mentioned above. In this case, a water based emulsion containing an acrylic resin is coated on the surface of the phosphor screen followed by drying. The "acrylic resin" mentioned includes acrylate resin copolymer and acrylate resin. It is necessary that the acrylic resin film formed be thick enough to fill the depressions of the rought surface of the phosphor screen so as to provide a substrate having a smooth surface upon which the metal layer is formed. But, the film should not be too thick. In the subsequent volatilizing step, voluminous gases are generated from the thick film owing to thermal decomposition of the acrylic resin film, causing blistering or cracks of the formed metal layer. Accordingly, in order to form a metal layer free from blistering and having a smooth surface it is necessary to control the thickness of the acrylic resin film appropriately. Such a control is difficult to achieve. Attentions should also be paid to the fact that the acrylic resin film is generally formed from an emulsion of acrylic resin particles. It follows that a large number of small depressions are observed on the surface of the acrylic resin film formed when the film is subjected to a microscopic observation. Obviously, the metal layer formed on the film can not help receiving influences from the irregular surface of the film.